This invention relates to a T-die adapted for an extrusion molding of a plastic material such as plastic film.
FIG. 1 shows a part of a conventional bending lip type of a T-die which comprises a lower stationary part S and an upper movable part M. The lower part S is provided with a steady lip portion 1 and the upper movable part M is provided with a flexible lip portion 4 and a base or neck portion 7, and a gap 2 of an elongated slit form is formed between the lower and upper lip portions 1 and 4. A plurality of die bolts 3 (3a, 3b, 3c, . . . ) are provided, usually screwed, for the upper projecting portion of the movable part of the T-die along the longitudinal direction thereof with equal spaces therebetween so that the die bolts 3 abut against and press downwardly the flexible lip portion 4 when the bolts 3 are driven downwardly.
According to the adjustment of the bolted amounts of the bolts 3 mutually or independently in a manual or automatical manner, the base portion 7 of the upper movable part M is bent or displaced to thereby obtain a desired thickness of the gap 2 of the elongated lip slit through which a plastic material is extruded. With the T-die having a construction described above, however, even when one specific bolt among the plural bolts 3 is adjusted to adjust only a specific portion of the gap 2, the influence of the adjustment of the specific bolt occurs to portions other than the specific portion which is to be really adjusted, particularly to the portions adjacent to the specific portion by the so-called mutual interference effect.
A graph of FIG. 2 represents deformed or displaced amounts .delta. of the flexible lip portion 4 of the T-die at portions of the flexible lip portion 4 corresponding to the locations of eight bolts 3a, 3b, . . . 3h and curves I, II and III represent mutual interference effects of the bolts 3 when the first, second and third bolts 3a, 3b and 3c are respectively bolted to downwardly press the corresponding portions of the flexible lip portion 4. As is apparent from FIG. 2, the adjustment of one bolt affects a considerably wide area of the flexible lip portion 4, i.e. thickness of the lip gap 2.
Although this deformation or displacement of the flexible lip portion 4 of the T-die does not provide very severe problems for the formation of usual non-elongated sheets or films of and the adjustment of the flow distribution of melted material passing through the lip gap, this displacement of the lip portion of the T-die will have relatively severe effects in specific cases as follows:
(a) In a case where it is required to adjust and control the edge profile of an original sheet adapted for a biaxially elongated film. PA1 (b) In a case where it is required to measure a position of a die-bolt for adjusting or changing the flow amount of a film passing through a specific portion of the flexible lip in use of a thickness gauge. PA1 (c) In a case where it is required to precisely or minutely adjust fine unevenness of a profile of a film passing through the flexible lip.
In order to obviate the problems or defects described before and to make it possible to deal with the above-described specific cases, the prior art provides an improved T-die such as shown in FIG. 3. The flexible lip portion 4 of the movable part M of the T-die shown in FIG. 3 is provided with a plurality of slits or narrow grooves 6 between the respective adjacent bolts 3 (3a, 3b, . . . 3h) to form a thin thickness portions of the flexible lip 4 to thereby weak the mutual interference effect of the bolts 3 when they are driven. The displacement amount .delta. of the lip portion according to the construction with the slits is represented by graphs shown in FIG. 4 with curves I, II and III under the same condition as that of the case shown in FIG. 2. These curves I, II and III show that a influence of the specific one bolt which is actually driven is transferred to a lesser degree than that in the case of the T-die shown in FIG. 1, particularly to the bolts 3e, 3f, 3g and 3h in the illustrated example.
As can be understood from the foregoing description, with the T-die adapted for an extrusion molding, the following two factors have to be always considered, one being continuity regarding the deformation or displacement of the flexible lip portion and the heat distribution, and the other being operation freedom or flexibility for operating the respective bolts with less interference effects between themselves.
It will easily be understood that the T-die having the construction illustrated in FIG. 1 which has a flexible lip portion provided with no groove has the best continuity, and regarding operation flexibility, the T-die shown in FIG. 1 is the worst. On the other hand, although not described in the drawings, a T-die of the type having divided lip portions corresponding to the respective adjusting bolts has the best operation flexibility and the worst continuity. Taking the above facts into consideration, it will be considered that the total characteristics of the T-die including the continuity and the operation flexibility are to be judged totally by taking into consideration the depth of the groove or slit formed in the flexible lip portion 4 of the T-die and the width of the individual groove or slit.
On this point, in a test performed with respect to the lip deformation or displacement profile, a curve VI in use of the T-die shown in FIG. 3 provided with narrow slits 6 represents a discontinuous pattern apart from a curve V regarding one relatively ideal continuous pattern in use of the T-die shown in FIG. 1. As understood from the above fact, in the prior art, although the mutual interference effect between the adjusting bolts 3 is weakened by providing narrow slits 6 for the flexible lip portion 4 of the movable part of the T-die, detailed analysis of the depth and the width of the slit 6 is not mentioned at all, which is very significant for the operational characteristics of a T-die of the type described hereinbefore.